Answer to Question #203095 in Astronomy | Astrophysics for Shridhi

Question

a) What is the so called cosmic reionization process?

(b) What are Lyα lines and why did the researchers want to observe them?

From this article attached link below

https://iopscience.iop.org/article/10.3847/2041-8213/ab75ec

Answer 1

a) Reionization is the process that caused the matter in the universe to reionize after the lapse of the "dark ages". Once objects started to condense in the early universe that were energetic enough to re-ionize neutral hydrogen. As these objects formed and radiated energy, the universe reverted from being composed of neutral atoms, to once again being an ionized plasma.

"The process that ionized the intergalactic medium (cosmic reionization) is expected to be spatially inhomogeneous, with fainter galaxies likely playing a significant role.", "The combination of local ionization at a redshift with substantially neutral bulk gas provides observational support for the picture of spatially inhomogeneous reionization that proceeds from the growth of ionized bubbles." (Tilvi et al., 2020)

b) Lyα line is the spectral line of hydrogen emitted when the electron falls from the n = 2 orbital to the n = 1 orbital. It is an important probe of the intergalactic medium and can be used to determine the frequency and density of clouds containing neutral hydrogen, as well as their temperature.

"The visibility of faint Lyα emission indicates an ionized IGM in the local vicinity of the group of galaxies". The emission of Lyα gave the opportunity to find the bubble of ionized gas near the cluster.